More than two years into the COVID-19 pandemic, scientists are still trying to understand why some survivors battle lingering symptoms.
Swedish researchers may have found a way to identify the people most susceptible to long COVID. Their findings may help scientists develop a blood test capable of predicting the people most likely to suffer from symptoms weeks or months after their initial infections.
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The researchers found people who developed long COVID had decreased levels of two antibodies, IgM and IgG3, that the immune system uses to fight infections. These antibodies usually increase with the threat of an infection.
When combined with a person's age, asthma history and COVID-19 symptoms, these factors were 75% effective at predicting long COVID, the researchers said.
"Overall, we think that our findings and identification of an immunoglobulin signature will help early identification of patients that are at increased risk of developing long COVID, which in turn will facilitate research, understanding and ultimately targeted treatments for long COVID," Onur Boyman, an immunology researcher at University Hospital Zurich, told The Guardian.
The most common lingering symptoms reported with long COVID are fever, shortness of breath, depression, myalgia and damage to the heart, lungs and vascular system, prior research has found.
A Penn State study found 1 in 5 COVID-19 survivors experience a decrease in mobility and almost 1 in 4 have concentration difficulties. Hair loss, rashes and digestive issues such as vomiting, diarrhea and loss of appetite also have been associated with long COVID.
Boyman said an estimated one-third of COVID-19 survivors may have lingering symptoms for at least a month.
In the study, researchers followed 134 patients for up to one year after their infections to see how their symptoms changed. They tested their predictive model against a group of 395 COVID-19 patients.
Charles Downs, an outside expert in long COVID at the University of Miami, told NBC News that in his experience, many long COVID patients tend to have either asthma or an allergy-related illness. He found the study's findings promising.
"There is no single test, no imaging study, that can be used to give a diagnosis," he said. "This helps move us in that direction."
The researchers acknowledged some limitations to their study, including a small sample size. It also was conducted before omicron became the dominant variant and didn't include vaccination status as a factor. Many of the study participants became ill before vaccines were available. Participants developed their initial infections between April 2020 and August 2021.
The researchers also knew which participants were suffering from long COVID at the start of the study.
The researchers said larger studies are needed to confirm their findings. The study was published in the journal Nature Communications.